1. Field of the Invention
This invention relates to an improved thermal ink jet printhead, and more particularly to a two part thermal ink jet printhead in which one part contains the ink flow directing channels, nozzles, and ink supplying reservoir, and the other part contains on a surface thereof the heating elements and ionicly passivated electronic driving circuitry therefor.
2. Description of the Prior Art
The printhead of U.S. Pat. No. 4,463,359 to Ayata et al discloses one or more ink-filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth and collapse of the bubbles.
U.S. Pat. Re. No. 32,572 to Hawkins et al discloses several fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. One part is substantially a flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves are formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be simultaneously made by producing a plurality of sets of heating element arrays with their addressing electrodes on, for example, a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer and, in one embodiment, alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks and then bonded together and diced into many separate printheads. A number of printheads can be fixedly mounted on a pagewidth configuration which confronts a moving recording medium for pagewidth printing or individual printheads may be adapted for carriage type ink jet printing. In this patent, the parallel grooves, which are to function as the ink channels when assembled, are individually milled as disclosed in FIG. 6B or anisotropically etched concurrently with the manifold recess. In this latter fabrication approach, the grooves must be opened to the manifold; either they must be diced open as shown in FIGS. 7 and 8, or an additional isotropic etching step must be included.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved thermal ink jet printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blowout, which causes ingestion of air and interruption of the printhead operation. In this patent, a thick film organic structure such as Riston.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble which is formed over the heating element to enable an increase in droplet velocity without the occurrent of vapor blowout.
U.S. Pat. No. 4,774,530 to Hawkins discloses an improved ink jet printhead which comprises an upper and a lower substrate that are mated and bonded together with a thick insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess which, when mated with the lower substrate, will serve as capillary-filled ink channels and ink supplying manifold, respectively. The grooves are open at one end and closed at other end. The open ends will serve as the nozzles. The manifold recess is adjacent the groove closed ends. Each channel has a heating element located upstream of the nozzle. The heating elements are selectively addressable by input signals representing digitized data signals to produce ink vapor bubbles. The growth and collapse of the bubbles expel ink droplets from the nozzles and propel them to a recording medium. Recesses patterned in the thick layer expose the heating elements to the ink, thus placing them in a pit, and provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess, so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,647,472 to Hiraki et al discloses a semiconductor device having an improved protective film and a process for producing it. The surface of a typical planar semiconductor device is covered with a protective film of an amorphous or polycrystalline silicon carbide which includes as an impurity at least one element selected from the group consisting of hydrogen, nitrogen, oxygen and a halogen. The protective film is formed by plasma CVD, propane, and a small amount of nitrogen monoxide, or it is formed by reduced pressure CVD with a reaction temperature of 800.degree. C. or more. In another embodiment, an additional insulating layer is formed over the protective film to increase the semiconductor device's ability to withstand voltage. This second insulating layer material may be Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, Nb.sub.2 O.sub.3, HfO.sub.2, Ta.sub.2 O.sub.3 or a low melting glass.
U.S. Pat. No. 4,686,559 to Haskell discloses a method of hermetically sealing an integrated circuit with a silicon nitride layer which is deposited directly on the surface to be sealed, followed by a second protective oxide layer. The nitride and oxide layers are concurrently patterned to expose the metallization for electrical contact. This results in a thinner nitride layer and an elimination of an entire set of masking, developing, and etching steps. The nitride layer should be 2000 to 6000 .ANG., and is deposited at a temperature of 300.degree. to 450.degree. C.
U.S. Pat. No. 4,699,825 to Sakai et al discloses a method of forming a silicon nitride film on a plurality of silicon wafers using a low pressure CVD process in which the wafer diameter may range from 100 to 150 mm while maintaining a relatively uniform film thickness. The CVD process is carried out under a pressure of 0.05 to 0.25 Torr and at a temperature ranging from 650.degree. to 1000.degree. C.
U.S. Pat. No. 4,298,629 to Nozaki et al discloses a gas plasma of a nitrogen-containing gas generated in a direct nitridation reaction chamber with the silicon body heated to a temperature of 800.degree. to 1300.degree. C. within the gas plasma to form a silicon nitride film on the silicon body. The resulting silicon-nitride film has a dense structure and low oxygen concentration than the prior art lower temperature process and a thick film is formed in a shorter period of time.
Japanese Laid-Open No. 61-135755 and published without examination on Jun. 23, 1986 to Watanabe discloses a thermal ink jet printhead having an array of heating elements and addressing electrodes formed on a substrate. The electrodes and heating elements are covered by a SiO.sub.2 layer. A light curable photosensitive layer is patterned and developed to form flow passage walls, and a glass plate is adhered to the walls to produce the printhead having a plurality of droplet emitting ink channels.
Japanese Laid-Open No. 61-291149 and published without examination on Dec. 20, 1986 to Katano discloses applying a fluorocarbon type water repellent to the ink jet printhead nozzle face. The vicinity of the nozzle face surrounding each nozzle is masked and a surfactant is applied to form an anti-static surface. This prevents adhesion of dust particles to the nozzle face and reduces the ink droplet ejection misdirection.
Printheads in carriage type printers must be reciprocally scanned across a recording medium, such as paper, so that a large number of droplet emitting nozzles requiring one lead per heating element per nozzle causes design and operating difficulty because of the large number of interconnections to the printhead with each of the many leads carrying high current. The necessity of making many interconnections increases printhead size and cost. Even in stationary pagewidth printheads, high lead count of one per nozzle results in an enormous number of leads for 300 pixels or spots per inch (SPI) and the industry is moving towards even higher printing resolution. It is not practical to wire bond pagewidth printers at 300 SPI when each heating element requires an associated wire bond. Thus, lead count reduction is enabling for compact pagewidth or scanning printhead architectures. Therefore, active integration of electronic circuitry on the heating-element-containing substrate to reduce the lead count is very economically attractive for high resolution printing (i.e., printing with a high jet or nozzle count).
There are two types of semiconductor devices which could be used for integration on the part of the printhead containing the heating elements; viz., bipolar and MOS (CMOS or NMOS). Bipolar devices exhibit thermal run away because device transconductance increases with temperature, so that conduction filaments are created, while the scattering mobile carriers from the channel surface of MOS devices degrades transconductance as temperature rises, leading to self shut down or self regulation of current over the total channel width of the device. Therefore, power MOS is inherently more suited for the thermal ink jet power switching application, especially where there is also uneven heating taking place, or where high currents are switched.
Power MOS also has a high switching rate because minority carrier recombination is not required to shut off the device, and it is relatively easy to produce 50 to 100 volt breakdown of drivers. High switching speed is important because the bubble generating resistors must be turned on and off in a few hundred nanoseconds. With bipolar devices, the minority carriers must recombine before turn off occurs. Also, power MOS is cheaper to manufacture and integrate with logic devices because no epitaxial wafer is required. The general industry trend is toward high use of MOS technology.
The single drawback of power MOS is sensitivity to mobile ions such as Na.sup.+, Li.sup.+, and K.sup.+, commonly found in the inks used by thermal ink jet printers. The sensitivity of MOS to ions is caused by the fact that mobile ions exist in SiO.sub.2 as charged species which drift under applied electric field, such as those created by a biased gate or metallization layer. The drifting of MOS devices results in unstable logic performance (shifting threshold voltage) and premature breakdown of high voltage devices.
Inks used for thermal ink jet printing have mobile ions as part of the dye species, and in any case, the ink manufacturing process produces inks which are quite impure by the standards of integrated circuits. The fact that the printhead temperature rises to about 60.degree. C. during use is also troublesome because mobile ion drift rate is accelerated by high temperature. Therefore, the electronic circuitry resident in the printhead must be protected from the ink and that is the subject of this invention.
Power MOS devices have increased sensitivity to mobile ions because a drift layer is present which does not have a field plate over it, only silicon oxide. If ions get into the silicon oxide above the drift layer, the field lines in the drift region become distorted. Breakdown will occur as a result of the distortion.
As printheads become more productive and produce more pages per minute, they need to last longer to have lower cost per page printed. Therefore, they need to last longer. Ionic passivation of the integrated electrical circuitry increases the printhead lifetime.